taru-adjective, adverb taking the 'to' particle
rugged; craggy; jagged
Formal or literary term used to describe steep, rugged mountains. Often appears in descriptive or poetic contexts.
峨々たる山々が連なっている。
Rugged mountains stretch in a chain.
峨々とした岩肌がそびえ立つ。
A craggy rock face towers above.
険しい is the common, everyday word for 'steep' or 'rugged', while 峨々 is a formal literary term used almost exclusively for mountains.
The kanji 峨 means 'high mountain' or 'steep', and the repetition 峨々 intensifies the meaning to 'very rugged'. The reading が is the on'yomi. The exact historical derivation is uncertain, but the form is a typical literary reduplication.